In Patent Document 1, a wireless power transmission system that does not use a power cable or a power transmission cable is proposed. That system has a power transmitter circuit with a power transmitting coil and a power reception circuit with a resonant circuit having a power receiving coil. The power transmitting coil causes AC magnetic field and the power receiving coil causes AC magnetic field, and these fields resonates. Power is transmitted wirelessly from the power transmitting coil in the power transmitter circuit to the power receiving coil in the power reception circuit. In Patent Document 1, the power reception circuit is composed of a power receiving resonant circuit that has a power receiving coil, a rectifier circuit, and a power storage device. The AC power is received by the power receiving coil in the power receiving resonant circuit, and the AC power is rectified to DC power by the rectifier circuit to charge the power storage device.
In Patent Document 2, a wireless power transmission system in which a DC voltage conversion circuit is set between the rectifier circuit and the power storage device to match impedances, in order to improve an impedance mismatch between the power transmitter circuit and the power reception circuit caused by impedance instability of the power reception circuit. In Patent Document 2, a chopper circuit is used as the DC voltage conversion circuit, and the impedance conversion ratio is adjusted by changing the duty ratio of the pulse signal for the chopper circuit.    Patent Document 1: JP2009-106136    Patent Document 2: WO2010/035321